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LT8311

8311f

For more information www.linear.com/LT8311

OPTO-COUPLER CONTROL

The LT8311 offers opto-coupler control to allow output

voltagefeedbackfromthesecondarytotheprimarysideina

forwardconverter.Usedinconjunctionwithaprimary-side

IC, the entire system offers fixed frequency peak current

mode control that has excellent line/load regulation and

quick transient response.

A basic understanding of the LT8311’s opto-coupler

control scheme can be obtained by referring to Figure 8.

The LT8311 senses the output voltage through a resistor

divider (RFB1 and RFB2) connected to its FB pin. The FB

pin voltage is compared to the lower of two inputs:

• An internal voltage reference of 1.227V

• Soft-start (SS) pin

At start-up, the SS pin capacitor, CSS, is charged up by the

LT8311’s internally trimmed 10µA current source. Since

FB tracks the lower of the SS pin and the 1.227V refer-

ence, the FB pin (and by extension the output voltage) is

forced to soft-start at the slew rate set by the capacitor,

CSS, connected to the SS pin.

NOTE: To ensure that the soft-start time of the converter

is controlled by the LT8311’s SS capacitor, CSS, it is

important to program the primary IC’s soft-start faster,

to get out of the way. If this is not done, the converter’s

soft-start time will be dominated by the primary IC’s soft

start, and the LT8311 will simply adjust its SS pin voltage

and slew rate to match the slower soft start time set by

the primary-side IC.

When the SS pin voltage gets higher than the 1.227V refer-

ence, the FB pin starts to track the 1.227V reference. The

output, therefore, regulates at a voltage set by the RFB1/

RFB2 divider network, and the FB pin’s regulation voltage

of 1.227V. The SS pin capacitor continues to get charged

up by the 10µA current source until it reaches its internal

clamp voltage of 2V.

OPERATION

Figure 8. The LT8311 Provides Voltage Feedback, as Part of a Peak Current Mode Control System, in a Forward Converter

VIN

VOUT

COUT

•

LOUT

–

VC IS ALSO

REFERRED TO

AS COMP IN SOME

PRIMARY-SIDE ICs

VREF

–

LT8311 OPTO CONTROL

–

140k

20k

COMP

1.227V

10µA

1.227V

–

0.9V

RFB1

CPL

RFB2

RLOAD

CSS

RSNS

GAIN

OPTO

8311 F08

–

MCG

MFG

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